High frequency antenna system



July 13, 1943. M. LEEDS ETAL HIGH FREQUENCY ANTENNA SYSTEM 2 Sheets-Sheet 1' Filed NOV. 15, 1941 lav.

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Inventors:

Laurance M. Leeds, m

T V. w M w m m C t 5 M M w WW a 2 July 13, 1943. '2

L. M. LEEDS ETAL 2,324,462 HIGH FREQUENCY ANTENNA SYSTEM 2 Sheets-Sheet 2 Filed Nov. 15, 1941 42 4'4 42 so FPEQUIA C'Y //V MFG'AZYC'ZIS Inventors: Leuu anc'e ["l. Leeds,

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Marvel W.Scheldor-fi y Their- Attorney.

' Patented July 13, 1943 HIGH FREQUENCY ANTENNA SYSTEM mane. M. Leeds, 1mm Junction. and

Marvel W. Seheldorf, Schenectady, N. Y., assigners to General Electric Company, a corporation of New York A Application November 15, 1941, Serial No. 419,234

, 12 Claims.

Our invention relates to a highfrequency antenna system and particularly to ahigh-fr equency directive antenna suitable for operation on horizontally polarized waves.

In broadcasting radio programs within the higher frequency channels, for example within the channels allotted at present by the Federal Communications Commission to frequency modulation service, it is generally desirable to distribute the energy radiated from the transmitting antenna as uniformly as possible in all horizontal directions. At the same time it is desirable to concentrate the radiated energy at low angles in the vertical plane. eflfective service, the radiating system should have a high degree ofvertical directivity and the horizontal field strength pattern should be as nearlycircular as'possible. It is also generally advantageous to polarize the transmitted waves horizontally rather than vertically for technical reasons familiar to those skilled in the art. Accordingly, it is an important object of our invention to provide an improved high-frequency antenna which possesses these and other desirable electrical characteristics. 3

In general, I simple two-terminal antennas In other words, for most It isparticularly an object of our invention to provide an improved and simplified antenna structure whichhas only two connection terminals and which nevertheless provides a substanheretofore known have possessed non-uniform radiation characteristics in the plane of polarization. Accordingly, it has heretofore been thought form horizontal wave pattern with horizontal polarization. Compound arrangements are obviously more expensive and more diilicult to adjust than the simpler radiating structures. For example, as the array becomes more complex, from the electrical standpoint it becomes increasingly diflicult to adjust the operating frequency, to match the impedances to the various circuit elements properly, and to secure the correct phase relationships between the currents in the active radiating elements. Furthermore, from the mechanical standpoint it becomes increasinglydifficult to provide the necessary support and insulation for the radiators and-transmission feed lines and to secure a strong, rigid construction capable of withstanding the weather conditions likely to be encountered in actual service. Accordingly, it is a further object of our invention to provide an improved and simplified antenna structure, capable of producing a substantially uniform radiation pattern in the plane of polarization, which is more economical to build and easier to adjust than prior art systems possessing comparable radiation characteristics.

tially circular radiation pattern in the plane of polarization.

Still another object of our invention is to provide an improved high-frequency directiv antenna which possesses these desirable characteristics and which, in addition, can be readily adjusted to any operating frequency withina wide range of frequencies without changing the physical dimensions of the active radiating elements.

Another object of our invention is to provide an improved high-frequency antenna system incorporating means for adjusting its feeding point impedance nearly independently of its natural frequency, within reasonable limits, without changing the physical dimensions of the active radiating elements.

The features of our invention which we believe to be novel are set forth with particularity in the appended claims. Our invention itself, however, together with further objects and advantages thereof, may best be understood by reference to ing a practical antenna structure embodying the invention; Fig. 4 is an enlarged fragmentary side elevation showing a modified construction at the transmission line feed point connections of the antenna of .Fig. 3; Figs. 5, 6a, 6b and '7 are raphs and schematic diagrams illustrating certain electrical characteristics of antennas constructed in accordance with the invention; and Fig. 8 is a schematic perspective vi'ew of a compound antenna; array having four unit sections constructed ating frequency and if no capacity exists between its free ends, then the current distribution along the dipole is sinusoidal essentially and that the current is maximum at its midpoint and zero at its ends. Such a current distribution is indicated by the dashed curve l2.

Assume next that capacity exists between the free ends of the dipole, as indicated by the capacity plates l3 and Id at the ends of dipole IS in Fig. 1b. Also assume that the dipole I5 is physically shorter than a half wave length by an amount such that the effective electrical length of the entire system is equal to one-half wave length. It is well known that the current at the free ends of the dipole is not zero, as in the case of Fig. 1a, but has some finite value. The dashed curve I6 indicates the current distribution along dipole l5 for a certain value of end capacity. It will be observed that the current distribution along dipole I5 is more uniform than in the case of dipole l0.

Assume next that the dipole l5 of Fig. 1b is formed into a planar loop with its free ends closely spaced in terms of the operating wave length. Such a structure is represented by the dipole H in Fi 2, the capacity between its free ends being represented by the capacity C1.. In accordance with our invention, this capacity is preferably made adjustable or variable for reasons that will shortly become apparent. Furthermore, it is sometimes desirable to provide a variable capacity C2 across the feed point connections l8. The function of this capacity C2 will also be considered at a subsequent point in this specification.

The instantaneous currents in the system of Fig. 2 are represented by arrows, in the same manner as in Figs. 1a and lb, and the current distribution for one value of C1 is represented by the dashed curve l9 (assuming that the eifective length of the antenna, including the effect of the capacities C1 and C2, is one-half wave length).

It will now be apparent that the dipole l1 and the capacities C1 and Ca in Fig. 2 provide a closed current loop for the high-frequency currents flowing in the system. In accordance with our invention the dipole I1 is always made shorter than a half wave length of the operating frequency, both physically and electrically, but the entire loop is made to appear equal to a half wave antenna at the feed points I8 through adjustment of C1 and C2. The field strength pattern in the plane of polarization, 1. e., in the plane of the loop, is always symmetrical. It has a maximum value along the axis AA and a minimum value along the axis B"-'B. For the theoretical case where the capacity C1 is equal to zero, the ratio of the field strength along the axis A-A to the field strength along theaxis B-B would be approximately equal to 1.4. However. in the system of Fig. 2 the capacity C1 simultaneously performs two important functions: (1) It controls the current distribution in the active radiator H as previously described, and (2) it determines the natural frequency of the system for any given physical dimensions of the loop and for any fixed value of C2. As the capacity Cl is increased, the current distribution, and consequently the radiation pattern in the plane of the loop, approach a circle, whichv is highly advantageous for high frequency broadcast service. Furthermorefor the same physical dimensions of the loop the larger the capacity C1, the longer is the effective electrical length of the system and the lower its natural frequency.

As the capacity C1 is increased, the radiation resistance of the system is, decreased. Conse-' quently, the feeding point impedance, which is proportional to radiation resistance, is likewise decreased. For example, in a. practical embodiment of the simple antenna schematically represented in Fig. 2 the impedance looking into the terminals l8 may be "only of the order of four to ten ohms. As a practical matter itis'difllcult to secure a'correct impedance match between such an antenna and atransmission line of reasonable dimensions. However, this disadvantage can be obviated and additional advantages, both electrical and mechanical, can be secured by utilizing a form of antenna construction shown in Fig.

3 and now to bedescribed.

The active radiating elements in the system shown'in Fig. 3 comprise two dipoles 20 and 2|. each of which is formed into the arc of a circle.

The circles through the center lines of these dipoles are of substantially equal diameters. The

dipoles 20 and 2| are positioned in substantially provide suitable terminal connections 22 and 23.

.Any suitable form of transmission line 24 known to the art may be utilized to effect transfer of energy between the feed connections 22 and 23 and high-frequency radio apparatus, represented only schematically by the box 25. In order to simplify the-drawings, the transmission line 2! has been represented ,asan open-wire line. In actual practiceit may be preferable to use a concentric transmission line provided with suitable means for effecting transition from unbalanced I to balanced condition; or in some cases it may be preferable t employ two concentric lines in balanced connection. It is believed that the details of these and other suitable transmission line structures will 'be readilyapparent to those skilled in the art. They have been mentioned only briefly for the sake of completeness.

The corresponding free ends of the dipoles 20 and 2| are conductivelyconnected together by the metallic membersj26 and 21. While it will be understood that any suitable'mechanical construction may be employed for the antenna structure thus far described, the dipoles 20 and 2| are preferably in the form of large tubular metal.

' The antenna systemof Fig..3-is supportedat a single point,.surrounding avertical supporting pole or mast 28, by means of aradialmember 29. The member'29 is rigidly secured tothe supporting pole 28' at oneend and to 'the-mid-point of the upper dipole 20 at-the other end. Both the supporting member 29 and the pole 28 areprefcrably of tubular metal construction. The mast 28 is preferably grounded to protect the transmission line 24 and high frequencyapparatus'fl against damage by lightning. ,The member 29 does not disturb the electrical performance of the system because the antenna current is a maximum at thispoint and the yoltage a mini- The ,lowcrdipole 2| is opened atits center to mum when the a correctly adjusted to the operating U-shaped plates 8| and ii are respectively fitted over the opposing faces of the blocks II and 21 in order to provide adjustable capacity surfaces between the ends of the dipoles 2| and 2|. As shown, each of the plates is provided a plurality of slots registering with screws threadedinto theblocks and. It will thus be apparent that the capacity betwen the plates assets:

maybeadiustedbylooseningthescrewsand moving the plates toward or away from each other. Of course any other suitable mechanical means may optionally be employed for varying or adjusting the effective capacity between the ends of the dipoles "and II.

ltwillberecogniaedthattheantenna structure of Fig. 3 resembles an antenna structure known to the art as a folded dipole, having its two arms bent into the are of a circle and provided with capacity plates II and II between its ends. The electrical characteristics of the folded dipole, in itsusual linear form, are'familiar to those skilled in the art of antenna design. Briefly, the radlationcharacteristics pf a folded dipole do not diifer substantially from the characteristics of a simple dipole of the same electrical length. The current distribution is approximately sinusoidal and the radiation pattern is essentially the same as the pattern produced by the simple dipole. Therefore, the system of Fig. 3 may be compared with the simplified system of Fig. 2, the pair ofdipoles ill and II corresponding to the dipole l1, and the capacitor 3., ll corresponding to the capacity 01. However, by employing the folded Wi e 0f dipole the principal disadvantage inherent in the system of Fig. 2, i. e., the low feeding point impedance, is overcome. c

The folded dipole performs the dual function of a radiator and an impedance matching transformer. The feeding point impedance presented to the transmission line by the folded dipole depends primarily upon the number of individual sections connected together and upon their physical dimensions. In the case-of a folded dipole having a plurality of sections of equal diameter the feeding point impedance is approximately equal to the impedance of one section alone multiplied by the square of the number of sections. Furthermore, the transformation ratio may be varied within limits by the use of conductors of unequal diameters as components of the radiating element. Thus in the system illustrated in Fig. 3 the diameter of the conductor 2. is

represented as being approximately twice the diameter of the conductor 2|. In a practical construction of the two-section antenna represented in Fig. 3 the feeding point impedance was somewhat greater than four times the impedance of one of the dipoles 2| or 2| alone, i. e., of the order of about'35 ohms. It is a relatively simple matter to match such an impedance to that of the transmission line. It will also be apparent in view of the foregoingthat a higher terminal impedance canhave been secured by employing three or more closely spaced sections in the folded dipole construction, 1. e., by adding one or more additional horizontal conducting loops with ends terminating at the connecting blocks "and 21. v

The radiation characteristics of the system of Fig. 3 are not substantially diiierent from those of the system of Fig. 2 previously discussed. If the peripheral length of the active antenna were the resonant frequency of the system overa very physically equal to one-half wave length, the

diameter of the loop would of course be theo-- reticlllly qual to about -159 times the wave length. However, as previously explained, due tothe end capacity Ci between the plates ll and l I, these values are actually somewhat less. For example, in the practical embodiment of the invention previously referredto, the active radiating element, comprising thedipoles 20 and 2|, was roughly one-third of a wave lengthlong and the loop diameter was consequently of the order of one-tenth of a wave length.

By varying the capacity 01 it is possible to vary wide range relative to its mean operating frequency. For example, Fig. 5 shows experimental reactance and resistance variation curves for an antenna similar to that of Fig. 3 except for the factthat both sections 20 and 2| were of the same diameter. The curves X1 and R1 show the reactance and resistance variations with frequency when the direct capacity between the plates II and II was about 20 micro-microfarads. It will be observed that the resonant frequency h of the system for this adjustment was about 39 megacycles. Similarly, the curves x2 and Rzshow the reactance and resistance variations with frequency when th direct capacity between these Plates was about 12 micro-microfarads. In this case it will be observed that the resonant frequency .f: of the system was about megacycles. In this particular system it was also .possible to vary the resonant frequency to somewhat higher values and to very much lower values, had this been desired. However, these curves illustrate that it is entirely practicable to operate an antenna system of this type, without changing the physical dimensions of the active radiating elements, at any frequency within the range which includes the present channels allotted to frequency modulation service, i. e., about 42-50 megacycles.

The curves of Fig. 5 also show that the radiation resistance of the system decreases as the resonant frequency is lowered by decreasing the spacing between the plates 30 and 3|. As previously described in connection with Fig. 2, variations in the current distribution, the horizontal field pattern and'the input impedance alsonecessarily occur at the same time. However, as a further refinement of our invention, the feeding point impedance of the system may be varied substantially independently of the radiation resist ance of the antenna, within reasonable limits, so

. as to offset this effect in a manner now to be described.

The curves X1 and X: of Fig. 5 indicate that the input reactance of this type of antenna varies rapidly with frequency. Therefore, conversely, a considerable reactance change is associated with a small change in the radiation resistance and natural frequency of the antenna. By virtue of this fact the feeding point impedance can be varied, within limits, by variation of a shunt capacity across the'input terminals of the system tance of the antenna alone.' This network may further be simplified to the form shown in Fig. 6b where R, L and C' respectivelyrepresent the equivalent series resistance, series inductance, and series capacitance of the entire system including C2. As C2 is increased from low values, the equivalent series network resistance R is caused to increase up to a limiting value equalto twice the actual antenna resistance Ra, without any serious change in the natural frequency of the system. The natural frequency of the system is easily readjusted to its previous value by readjustment of capacity C1. The curve of Fig. '7, which was prepared from theoretical considerations, shows how the ratio of R1 to Ra varies with change in C2. As C2 is increased, the ratio R to R4. increases up to a limiting value of 2, corresponding to the maximum value which still permits a resonant condition in the antenna circuit.

The portions of the curve of Fig. 7 having a negative slope show that the same values of C: may be used to give an equivalent network resistance R much greater than 2Ra. These conditions are equivalent to shunt resonance in the system. Thus for a given value of C2, for example as indicated by the dashed line 50 in Fig. '7, the point corresponds to the resonant frequency of the system for equivalent series resonance and the point 52 corresponds to the resonant frequency of the system for equivalent shunt resonance. However, in the latter case a variation in C2 produces a very'much greater change in the natural frequency. Therefore the system should always be operated in the range of equivalent series resonance to prevent material variation in the resonant frequency of the system as C: is varied.

In order to secure greater vertical directivity and greater gain in the horizontal plane, a plurality of antenna units may be arranged in a tier,

spaced apart equally along a common vertical axis. Such a compound antenna array having four unit sections 60, Si, 62, and 83 constructed in accordance with the invention is schematically represented in Fig. 8. The units are supported in any suitable manner, not shown, preferably by a single central mast extending along the axis D-D of the system in the same manner as the antenna of Fig. 3. For maximum directivity in the horizontal plane, the vertical spacing between the adjacent sections is proportioned in accordance with the principles set forth in Patent 2,254,697, Sidney Godet, assigned to the same assignee as the present invention. It so happens that for this particular type of antenna system, the optimum spacing between units for maximum directivity in the horizontal plane is very close to one wave length. Accordingly, where the power required to be radiated from the system is not too high, all of the units may be cophasally energized from the high frequency apparatus 10 through a common transmission line structure in the manner shown in Fig. 8. A

balanced feed is provided by the two concentric transmission lines 14' and By employing quarter wave matching sections ".01, II, and it between the respective antenna units and the transmission feed imesuie m1 impedance presented to the on line by each unit m y be increased to reasonable values. For compound arrays operating; at high power it will Sgenerally be preferable to employ a plurality of separate transmission lines from, each unit antenna to a common Junction box, as will readily be appreciatedby those skilled in 'tbe'art.

The foregoing description h'ss,shown that antenna structures embodying our invention possess many advantagesfboth electrical and me--' chanical, over prior art systems. Many other advantages are secured. These antennas are dimensionallysmaller than prior art structures of similar radiation characteristics, present less area for wind'load and have a pleasing appearance. They are also readily adapted to sleet and ice melting equipment, since heating elements may be included within the radiators and connections may bemade through the grounded tubular mast and radial support. By employing a solid dielectric material between the plates of the capacitors C1 and C1, the system can be made relatively insensitive to the eifects of rain, snow,

and sleet, which otherwise tend to vary the resogant frequency by causing variations in C1 and It has heretofore been proposed to use apair of horizontal half-wave dipoles, crossed at right angles and energized in phase quadrature, in order to secure a substantially circular radiation pattern in the horizontal plane. The antenna of Fig. 3 has reduced radiation in directions above and below the horizontal p1ane, and consequent- 1y greater field strength in the horizontal plane for a given power input, as compared to this prior art antenna. Furthermore, in a compound array embodying our invention, such as is shown in Fig. 8, the antenna units are necessarily more widely spaced for optimum directivity in the horizontal plane, because of the increased horizontal directivity of theindividual units Consequently, since .there is less coupling between units, the adjustment of the individual'units in the array is simplified.

Although our invention finds particular application in a transmitting antenna system, and althoughthe foregoing specification has been presented with emphasis on that fact, it will of course be apparent that our improved antenna system may be employed with receiving equipment. The fact that the directivity, current distribution and other characteristics of a particular antenna are substantially the same whether it is employed to abstract energy from an incident high' frequency electromagnetic wave or to radiatesuch' a waveisso well known as to require no elaboration. I

It will also be apparent that, while we have shown and'described whatwe believe to be preferred embodiments of our invention, many other modifications will readily sugg'estthemselves to those skilled in thevart. "For example. the active radiating element need not be in the shape of a circle, although this form is preferred because of its pleasing appearance and simple construction. The essential requirement in this respect is that the antenna comprises a peripherally incompleteconducting loop enclosing a substantial spaced capacitive relationship. The radiation pattern produced by a loop in the form of a square or other similar p ly on differs only very slightly from that of the circular loop. Also, while the dipoles a and ii have been represented in Fig. 3 as being positioned one above the other,

the electrical characteristics 'of the system are not substantially diii'erent if the dipoles have slightly different diameters and are both positioned in the same horizontal plane so long as they lie on substantal y parallel curves'and are closely spaced from each other in terms of the operating wave length. We therefore containplate by the appended claims to cover any such modifications as fall within the true spirit and scope of our invention. I 1

What weclaim as new and desire to secure by) Letters Patent of the United States, is: 1 y

1. A high frequency antenna system compris-1 ing a plurality of closely spaced, separated conductors similarly oriented and connected together at their corresponding ends and forms 3 inga peripherally incomplete loop, theends of said loop being closely spaced and terminating in means, providing substantial capacity thereof said loop being closelyspaced and terminating in means providing substanital l pooity' therebetween, one of said conductors being coupled to a transmission line for transfer of high-frequency energy between said loop and said line, said loop being physically shorter than one-half wave length at frequencies within said range, and means for varying said capacity, thereby. to adjust said antenna to resonance at any desired fre quency within said range.

3. A high-frequency directive antenna system adapted to be tuned to any frequency within a relatively wide range of operating frequencies comprising a plurality of conductors each in the shape of a peripherally incomplete planar loop, said conductors being positioned in substantially parallel relationship and closely spaced from each other, a pair of capacity surfaces, one end of each conductor being connected to one of said surfaces and the other end of each conductor being connected to the other of said surfaces-said conductors being physically shorter than a half wave length at frequencies within said range, a transmission line coupled to one of said conductors at its midpoint, and means for varying the effective spacing between said surf aces, thereby to adjustsaid antenna to resonance at any desired frequency within said range. I

4. A high-frequency directive antenna system comprising a plurality of closely spaced, separated conductors similarly oriented and connected-together at their corresponding ends and forming a peripherally incomplete loop, the ends of said loop being closely spaced and terminating in means providing substantialcapacity therebetween, one of said conductors also being open minals, aonnne eonnectedtosaidterlninals for transfer of high frequency energy between said loop and said line, and means for "W s the terminal impedance presented to said line by said system, said t means comprising a variable capacitor connected between said terminals, said system being adjusted to have an eifective electrical length substantially equal to one-half wave length at said highfrequency.

5. A highfrequency directive antenna system adapted to be tunedto any frequency within a relatively wide range of operating frequencies comprising a plurality of closely spaced substantially parallel dipoles connected together at their correspo ding ends and forming a peripherally incompl te loop, the ends of said loop being closely spaced and terminating in variable lusted to be resonant at capacity means, one of said dipoles also being open at its midpoint to provide a pair of terminals, a transmission line connected to said terminals 1 for transfer of high-frequency energy between said loop and said line, said loop being physicallyshorter than one-half wave length at frequencies within said range, and second means providing variable capacity between said terminals, whereby through adjustments of said variable capacity means, said system may be adany desired frequency within said range andto havea predetermined i terminal impedanceat said desired frequency.

6. A directive antenna system for operation at a high frequency comprising a plurality of dipoles each in the shape'of a peripherally incomplete planar loop, said dipoles being positioned in substantially parallel relationship and closely spaced from each other in terms of a wave length'at said highfrequency, a pair of capacity surfaces closely spaced from each other of said conductors at its in terms of said wave length, one end of each dipole being conductively connected to one of said surfaces and the other end of each dipole being conductively connected to the other of said surfaces, said dipoles being physically shorter than a halfwave length at said frequency, and a transmission line electrically coupled to one of said dipoles.

7. A high frequency directive antenna system for radiating horizontally polarized waves comprising a plurality ofclosely spaced, substantially parallel conductors connected together at their corresponding ends and forming a peripherally incomplete horizontal loop. the en of said loop terminating in a pair of surfaces having substantial capacity therebetween, .a source of high frequency energy, means for energizing said loop. from said source comprising a transmission line electrically coupled to one midpoint, said system having an effective electrical length substantially equal to one-half wave length at the operating frequency, the configuration of said loop and the value of said capacity being so proportioned that the horizontal field strength pattern produced by said system approaches a circle. 8. A high frequency directive antenna system comprising a plurality of closely spaced, separated conductors similarly oriented and connected together at their corresponding ends a and forming a peripherally incomplete loop, the

at an intermediate point to provide a pair of terends of said loop being closely spaced and terminating in means providing substantial capacity therebetween, one of said conductors having center feed terminalsconnected to a transmission line for transfer of high frequency energy between said loop and said line, said system separated conductors similarly oriented and connected together at their corresponding ends and forming a peripherally incomplete horizon- I may,

tal loop, the ends of said loop being closely spaced I and connected to the opposite sides of a variable capacitor, one of said conductors being open at its midpoint to provide a pairof terminals, a transmission line connected to said terminals, a second variable capacitor connected between said terminals, said loop being physically shorter than a halt wave length at the operating irequency of the system and tuned to resonance at said frequency by adjustments of said capacitors, and a conducting supporting for said systemattached to another of said conductors at its midpoint.

10. A transmitting antenna for radiating horizontally polarized high-frequency waves substantially uniformly in all horizontal directions comprising, in combination, a plurality of substantially circular horizontal radiators, said radiators being circumferentially incomplete, similarly'oriented with respect to a common vertical axis and closely spaced from each other in terms of the operating wave length, said radiators being connected together at one end by a first capacity plate and at the other end by a second capacity plate which is closely adjacent to said first plate, one 0! said radiators also being open at its midpoint, a transmission feed line connected to said midpoint, said radiators being physically shorte than a half wave length at said frequency, and means for adjusting said antennat resonance at said frequency comprising means to adjust the capacity between said plates.

11. A transmitting antenna for radiating horizontally polarized high frequency waves substantially .uniiQ ly in all horizontal directions ,comprising in combination a plurality of radiators lying i 'th a cs or concentric horizontal circles,said radiators beingclosely spaced from each otherand similarly oriented about the common vertical axis,, saidiradiators' being eonnected together atone end we first capacity plate and at the otherend by a second capacity plate which isiclosely adjacent to saidfirst plate, a adapted, to supply, current oi a high irequency from a source to said antenna, one of said radiators having. center feed terminals connected to said transmission line, said radiators physically' shorter thana hall wave length at said high frequency, and means ior adjusting said antenna to resonance at said frequency comprising} to adjust the capacity between said plates,

12. A high ire'quency' directive antenna system for radiating horizohtallypolarized waves comprising a plurality Y of radiators lying substantially in the arcs of concentric horizontalfcircles,

said radiators being closely spaced from each other and similarly oriented about the common vertical axis, a variable capacitor having afpair oiplates, one end of 'eachradiator terminating in aconnection to one of said plates and the other end of each radiator terminating .in' a connection to the other of 'said'plates, one of said radiators being open at its center to provide a' pair of inputterminals, a transmission feed line to said'terminals, a second variable capacitor connected across said terminals, said radiators being physically shorter than 'a half wave length at'theoperating frequency of 1 the system and tuned to resonance at said'i're quencyby adjustments oi'said' variable capaci tors, and a single point support for said system comprising a conductive supporting member attached toanother of said radiatorsat itsmidpoint. y

I LAURANCE M. LEEDS.

MARVEL W. SCHEIDQRF. Y 

